I went about doing this at the end of the mission "Servant of the Machine".
There will be a part at the end of the mission where you're supposed to hold position around this thing. During this, waves of enemies will spawn in around 10-15%, 33%, 50% and lastly the lesser sorcerer will spawn in the final wave at the 75% mark. Standing around the objective increases it, while standing outside does not, but also doesn't decease so there's no penalty for just killing everything as it comes along.
That brings us to the last wave. I'd always save the Melta Bomb for one of the groups of 2 enemies that come with the sorcerer, then I'd switch to a nearby Krak grenade and when the sorcerer was in his respawn animation I lobbed two over at him, they auto stuck to him, blowing him up and awarding the achievement. Best part is there's a checkpoint before this section so if you mess up just reload the checkpoint.